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feat(linux): Add secure boot using U-Boot verified boot for AM62LX
Add a new page in the U-Boot user guide documenting secure boot enablement using U-Boot's verified boot framework in AM62LX. For AM62LX, u-boot.img signing and verification has been moved to leverage the mechanisms provided in U-Boot from using HSM core in other K3 devices. This page aims to address this change in AM62LX. Signed-off-by: T Pratham <t-pratham@ti.com>
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‎configs/AM62LX/AM62LX_linux_toc.txt‎

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@@ -37,6 +37,7 @@ linux/Foundational_Components/U-Boot/UG-Memory-K3
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linux/Foundational_Components/U-Boot/UG-UMS
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linux/Foundational_Components/U-Boot/UG-QSPI
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linux/Foundational_Components/U-Boot/UG-UART
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linux/Foundational_Components/U-Boot/UG-Secure-Boot
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linux/Foundational_Components/U-Boot/UG-Key-Writer-Lite
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linux/Foundational_Components/U-Boot/UG-Programming-OTPs
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‎source/linux/Foundational_Components/U-Boot/UG-Secure-Boot.rst‎

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.. _u-boot-secure-boot-verified-boot:
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################################################
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Secure boot using U-Boot verified boot framework
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################################################
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The complete Secure Boot documentation is available at:
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:ref:`foundational-secure-boot`. This page specifically covers the
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authentication and verification of U-Boot image using U-Boot verified boot.
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On most other K3 devices, signing and verification of all boot binaries takes
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place in the Hardware Security Module (HSM), and thereafter U-Boot hands off
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the secure chain of trust to the Linux kernel :file:`fitImage`.
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On AM62Lx, we have transitioned to use the native U-Boot secure boot framework
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for a part of this chain of trust. The U-Boot documentation covers more theory
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on this at
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`U-Boot Verified Boot <https://docs.u-boot.org/en/latest/usage/fit/verified-boot.html>`__
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and `U-Boot FIT Signature Verification <https://docs.u-boot.org/en/latest/usage/fit/signature.html#signed-configurations>`__.
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The thing to note is, we are applying the same concepts to U-Boot FIT as the
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kernel FIT examples in the preceding links.
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The verification of :file:`tiboot3.bin` and :file:`tispl.bin` is still handled
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inside the HSM. However, we hand off the chain of trust to U-Boot just after
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this. The :file:`u-boot.img` is a signed FIT image. The U-Boot SPL binary
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embeds the public key derived from the private key used to sign the U-Boot FIT.
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This is used to verify the authenticity of the loaded U-Boot binary by the
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U-Boot SPL.
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**************
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The FIT source
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**************
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The U-Boot FIT configuration node contains a signature sub-node.
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.. code-block:: dts
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conf-0 {
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description = "k3-am62lx-evm";
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firmware = "uboot";
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loadables = "uboot";
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fdt = "fdt-0";
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signature {
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algo = "sha512,rsa4096";
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key-name-hint = "custMpk";
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sign-images = "firmware", "loadables", "fdt";
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};
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};
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It specifies the key name and algorithm to use for signing, and the images
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that need to be signed.
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The public key is similarly embedded into U-Boot SPL using a binman property
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called :code:`u-boot-spl-pubkey-dtb`. This handles the heavy lifting of calling
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the appropriate mkimage commands and packing the public key in the SPL DTB
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correctly.
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.. code-block:: dts
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tispl.bin {
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...
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spl: section {
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u-boot-spl-nodtb {
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};
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u-boot-spl-pubkey-dtb {
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algo = "sha512,rsa4096";
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required = "conf";
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key-name-hint = "custMpk";
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};
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};
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};
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The :code:`key-name-hint` property in both these nodes searches for the
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:file:`custMpk.key` private key and :file:`custMpk.crt` public key certificate
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in the directories defined in the :code:`BINMAN_INDIRS` variable. The default
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TI dummy keys are located at :file:`arch/arm/mach-k3/keys/` and are copied at
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the start of the build by binman into the build directory:
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.. code-block:: dts
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custMpk-crt {
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filename = "custMpk.crt";
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custmpk_crt: blob-ext {
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filename = "arch/arm/mach-k3/keys/custMpk.crt";
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};
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};
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custMpk-key {
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filename = "custMpk.key";
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custmpk_key: blob-ext {
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filename = "arch/arm/mach-k3/keys/custMpk.key";
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};
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};
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********************
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Runtime verification
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********************
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At runtime during device boot, U-Boot SPL loads the :file:`u-boot.img` and then
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verifies the FIT signature using the public key it has in its DTB. If the
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verification passes, boot continues. Otherwise, the boot is aborted.
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***********************
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Changing the dummy keys
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***********************
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The SDKs use the TI dummy key for signing the U-Boot FIT image. But you may
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want to use your own key for testing and production. For this, replace the
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:file:`arch/arm/mach-k3/keys/custMpk.key` and
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:file:`arch/arm/mach-k3/keys/custMpk.crt` with your own key and crt files. The
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filenames need to be the same.
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It is also possible to use your own keys located at a different location. You
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need to change the complete path in the :code:`filename` property above in
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:code:`custMpk-crt` and :code:`custMpk-key` in
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:file:`arch/arm/dts/k3-am62l3-evm-binman.dtsi` to your .crt and .key files.
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After either of the above changes, the U-Boot needs to be built again to get
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the signed binaries with the updated keys. Refer to :ref:`top-level-makefile`.
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.. note::
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Generating a new set of keys:
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.. code-block:: console
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$ mkdir keys
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$ cd keys
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$ # Generate an RSA private key:
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$ openssl genpkey -algorithm RSA -out custMpk.key \
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-pkeyopt rsa_keygen_bits:4096 -pkeyopt rsa_keygen_pubexp:65537
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$ # Build your cert template (Enter necessary details in the prompts that follow):
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$ openssl req -new -key custMpk.key -out cert.csr
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$ # Self-sign the certificate
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$ openssl x509 -req -days 3650 -in cert.csr -signkey custMpk.key -out custMpk.crt

‎source/linux/Foundational_Components/U-Boot/Users-Guide.rst‎

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UG-AVS
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UG-Thermal
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UG-Splash-Screen
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UG-Secure-Boot
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UG-Key-Writer-Lite
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UG-Programming-OTPs
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UG-Falcon-Mode

‎source/linux/Foundational_Components_Secure_Boot.rst‎

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.. ifconfig:: CONFIG_part_variant in ('AM62LX')
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The U-Boot's Secondary Program Loader (SPL) securely verifies the U-Boot
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proper. U-Boot uses its verified boot framework to do this. U-Boot proper
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then securely verifies and decrypts the kernel, Device Tree Blobs (DTB),
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and initramfs.
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proper. U-Boot uses its verified boot framework to do this
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(See: :ref:`u-boot-secure-boot-verified-boot`). U-Boot proper then securely
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verifies and decrypts the kernel, Device Tree Blobs (DTB), and initramfs.
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.. Image:: /images/AM62L_KF.png
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:scale: 70%

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